This invention relates to apparatus and methods for the analysis of gold and related metals, in particular to an apparatus and method using an ion-interaction reverse phase liquid chomatographic (RPLC) method and apparatus that includes the use of a concentrator column as a means of increasing the limits of detection of the gold and related metals, whilst effectively eliminating matrix interferences.
The art is replete with a variety of methods for the analysis of gold, particularly in ores and process streams. The major methods used are spectroscopic and electrochemical, with atomic absorption spectroscopy using either the flame technique for the determination of gold in ppm concentrations or the carbon furnace for much lower concentrations being the most common method. However, even when using carbon furnace atomic absorption spectroscopy, it is usual to preconcentrate a sample using solvent extraction.
The use of cyanide in the recovery of gold from its ores is by far the most widely used method of gold extraction. One advantage in the use of cyanide is that it frequently permits low grade ores to be effectively treated.
In the cyanide process, an ore is agitated with excess cyanide in the presence of oxygen. Typically, 0.02 -0.08 percent cyanide is reacted with the ore for up to 72 hours with lime being added to neutralise acidic ore components and maintain an alkaline pH. In this reaction, gold is solubilised as gold (I) cyanide whilst other metals that are commonly present are also oxidised and solubilised as a variety of cyano complexes. It is usual that these other metal cyano complexes are present in solution at concentrations much greater than the gold.
One commonly used method for the recovery of gold from these cyanide solutions is the carbon-in-pulp (CIP) process. In this process, the leachate containing aurocyanide in preference to other gold cyano complexes, is passed in a counter-current direction to tanks containing activated carbon, emerging as barren leachate containing only low levels of gold of the order of 10 ppb, that is, 10 parts per 10.sup.9 parts. The barren leachate is treated to become tailings, whilst the loaded carbon is firstly washed to remove contaminant metal cyano complexes and then aurocyanide is stripped using relatively concentrated alkaline cyanide solutions. Gold may then be recovered electrolytically therefrom, the carbon being reactivated and returned to the process.
To assess the performance of the CIP process, the concentration of gold present at various stages of the process may be monitored. Generally, the concentration of gold will vary from low ppm in the cyanide leachate to low ppb in the barren leachate and tailings. It is to be noted that the most important indicator of process efficiency is obtained from the analysis of these latter solutions.
The present inventor has previously shown that it is possible to analyse cyanide leachate for gold at low ppm levels using ion-interaction RPLC. This method is described in J. Chomatography 361 141 (1986). The essence of this method is the use of a reverse phase C.sub.18 column and as eluent, acetonitrile-water in the range 23:77 to 30:70, v/v containing 5 mM low UV PIC A.
It was found that this method had three inherent advantages. Firstly, it was specific for aurocyanide which is the most important form of gold for recovery using the CIP process, secondly by optimizing this method, the present inventor found that 40 ppb (0.04 ppm) was the limit of detection and thirdly, other metal cyanide complexes, platinum, palladium and silver could be determined.
However, this method was not suitable for the monitoring of the barren leachate or tailings due to an insufficiently low limit of detection.
Because of the importance of the gold levels in the barren leachate and tailings in the monitoring of the efficiency of a CIP process, the present inventor has recognised that there exists a need for a gold and related metal analytical method that is capable of determining gold when present at sub ppb levels.